This invention relates to an improved process for dyeing a textile fabric which contains shrinkable fibers, and more specifically, to a particularly advantageous and desirable process for beam dyeing polyester textile fabrics, especially polyester knit fabrics. The invention relates also to the dyed fabrics produced by this process.
Fabrics formed of polyester fibers, and polyester knit fabrics in particular, are susceptible to a comparatively high degree of shrinkage when the fabric is heated to elevated temperature during dyeing. Polyester knit fabrics are also highly subject to distortion and longitudinal stretching in the wet processing steps associated with dyeing.
A common method of dyeing polyester knit fabric is by jet dyeing. In jet dyeing, the fabric is handled in a rope-like manner as it is directed through the dye bath. The longitudinal pulling and stretching of the rope of fabric during processing causes a reduction in the width of the fabric, and this, together with the relatively high temperatures required for dyeing polyester fibers, contributes to substantial shrinkage of the fabric in the widthwise dimension. As a result, when the fabric is removed from the jet dyeing machine, it must be stretched widthwise on a tenter frame in order to bring the fabric back to the width desired in the finished fabric. During this tentering operation, the fabric is directed through a heating zone to effect heat setting of the fabric. It has been recognized that this type of heat setting imparts an undesirably harsh hand to the fabric, and it is usually necessary to subject the fabric to further mechanical surface treatment operations, such as napping, shearing, tigering, polishing and/or sanding, in order to alleviate the harsh hand and provide a fabric which is commercially acceptable. In some cases, such as in delicate knits for lingerie for example, the stretching and heating treatments necessary after jet dyeing may so alter the fabric properties as to render the fabric unsuited for its intended purpose. In addition to the disadvantages and limitations noted above, jet dyeing is restricted to relatively small dye lots, which thereby increases labor and energy costs, and increases the difficulty in producing uniform dye lots. Additionally, the very nature of the jet dyeing process involves numerous processing variables which make it quite difficult to obtain reproducible dye lots. More specifically, the jet dyeing process typically involves the simultaneous processing of several ropes of fabric through the jet dyeing apparatus. In order to achieve reproducible results within a given dye lot and from lot to lot, the ropes of fabric undergoing dyeing must be of equal weight, they must be traveling at the same speed through the machine and subjected to the same hydraulic forces, and they must receive the same amounts of dye liquor.
It has been recognized that the dyeing of textile fabrics by beam dyeing has certain advantages over the jet dyeing process described above. In beam dyeing, the fabric to be dyed is wound in open width on a perforated cylinder or beam for dyeing in a closed vessel. Not only does beam dyeing produce a more uniform or homogeneous color throughout a batch of fabric, and from one batch of fabric to the other, but beam dyeing permits dyeing of the fabric in larger batches, and is therefore a more economical process.
However, beam dyeing is used commercially mainly for dyeing relatively stable fabrics. Beam dyeing has not been successfully used heretofore for the dyeing of relatively unstable shrinkable fabrics formed of highly shrinkable fibers, such as polyester. One of the primary problems encountered in attempting to dye such fabrics by the beam dyeing method is their relative instability and the large degree of shrinkage which occurs during dyeing. At the temperature typically used for dyeing polyester fabrics, for example about 130.degree. C., the fabric would shrink and contract about the dye beam to such an extent that the dye discharged through the beam could not penetrate through the thickness of the layers of fabric wound on the beam, producing uneven color and an unacceptable product.
In an effort to overcome these problems and make it possible to utilize beam dyeing for such fabrics, various attempts have been made to stabilize the fabric prior to the beam dyeing operation using treatment methods which would be expected to limit the amount of shrinkage during dyeing, such as for example by shrinking in a liquid bath or on a tenter frame. However, such prior efforts have been unsuccessful.
With the foregoing in mind, it is an object of this invention to provide a method by which textile fabrics formed of highly shrinkable fibers, such as polyester, can be successfully dyed by the beam dyeing method.